The present invention relates to a speed change transmission apparatus including a hydrostatic stepless speed change section receiving an output of an engine, a planetary transmission section having a plurality of planetary transmission mechanisms and a plurality of output members, the planetary transmission section being configured to combine/synthesize a drive force outputted from the hydrostatic stepless speed change section and an engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section, and a speed change output section having an output shaft, the speed change output section being configured to output combined drives force outputted from the plurality of output members in a plurality of different speed ranges from the output shaft.
Next, in order to explain the advantage of the present invention, the construction of the invention filed for a patent application as Japanese Patent Application No. 2005-286073 and assigned to this applicant will be explained in comparison with the present invention. It should be noted; however, that the following discussion is not to be taken as this applicant's acknowledgement of the compared construction as prior art.
FIG. 24 is a diagram of a speed change transmission apparatus for comparison. As shown in this figure, the speed change transmission apparatus includes a hydrostatic stepless speed change section 20, a planetary transmission section 3a and a speed change output section 3b. The planetary transmission section 3a includes a first planetary transmission mechanism P1, a second planetary transmission mechanism P2 and a third planetary transmission mechanism P3. The first planetary transmission mechanism P1 includes a ring gear which receives an engine drive force not subjected to any speed change action by the hydraulic stepless speed change section 20 and a sun gear which receives the output from the hydraulic stepless speed change section 20. The second planetary transmission mechanism P2 includes a carrier operatively coupled to the ring gear of the first planetary transmission mechanism P1 and a ring gear operatively coupled to a carrier of the first planetary transmission mechanism P1. The third planetary transmission mechanism P3 includes a sun gear operatively coupled to a sun gear of the second planetary transmission mechanism P2 and a carrier operatively coupled to a ring gear of the second planetary transmission mechanism P2.
The speed change output section 3b includes a clutch section C which receives the output of the planetary transmission section 3a and an auxiliary speed change section 100 which receives the output of this clutch section C. The clutch section C includes a first clutch C1 and a second clutch C2 which have input portions thereof coupled respectively to a pair of output portions of the planetary transmission section 3a. The auxiliary speed change section 100 includes a low speed clutch CL and a high speed clutch CH disposed between an input shaft and an output shaft of this auxiliary speed change section 100.
FIG. 25 is an explanatory figure illustrating relationship among speed change conditions of the hydrostatic stepless speed change section 20, speed ranges and output speeds of the auxiliary speed change section 100 (“auxiliary speed change outputs” hereinafter) of the speed change transmission apparatus 3. The sign “−MAX” shown in FIG. 25 denotes the maximum speed change condition under a reverse rotation transmission condition of the hydrostatic stepless speed change section 20. The sign “N” denotes the neutral condition of the hydrostatic stepless speed change section 20. The sign “+MAX” denotes the maximum speed change condition under a forward rotation transmission condition of the hydrostatic stepless speed change section 20. FIG. 26 is an explanatory figure illustrating relationship between speed rages and operational conditions of the clutches of the speed change transmission apparatus 3. The sign “ENGAGED” shown in FIG. 26 represents engaged conditions of the respective clutches C1, C2, CL, CH. The sign “−” represents disengaged conditions of the respective clutches C1, C2, CL, CH.
When the first clutch C1 and the low speed clutch CL are operated to the engaged conditions and the hydrostatic stepless speed change section 20 is speed changed from “−MAX” toward “+MAX”, the speed change transmission apparatus 3 is speed changed to the first speed range and the rotational speed of the auxiliary speed change output is increased steplessly from “0”. When the hydrostatic stepless speed change section 20 reaches the speed condition “+MAX”, the rotational speed of the auxiliary speed change output becomes “B11”. In association with this, as the second clutch C2 and the low speed clutch CL are operated into the engaged conditions and the hydrostatic stepless speed change section 20 is decelerated from “+MAX”, the speed change transmission apparatus 3 is speed changed into the second speed range and the rotational speed of the auxiliary speed change output is steplessly increased from “B11”. When the hydrostatic stepless speed change section 20 is operated to a speed change condition ‘A’ which is between “+MAX” and “N”, the rotational speed of the auxiliary speed change output becomes “B12”. In association with this, as the first clutch C1 and the high speed clutch CH are operated into the engaged conditions and the hydrostatic stepless speed change section 20 is accelerated from “A”, the speed change transmission apparatus 3 is speed changed into the third speed range and the rotational speed of the auxiliary speed change output is steplessly increased from “B12”. When the hydrostatic stepless speed change section 20 reaches the speed condition “+MAX”, the rotational speed of the auxiliary speed change output becomes “B13”. In association with this, as the second clutch C2 and the high speed clutch CH are operated into the engaged conditions and the hydrostatic stepless speed change section 20 is speed changed toward “−MAX”, the speed change transmission apparatus 3 is speed changed into the fourth speed range and the rotational speed of the auxiliary speed change output is steplessly increased from “B13”. When the hydrostatic stepless speed change section 20 reaches the speed change condition “−MAX”, the rotational speed of the auxiliary speed change output becomes the maximum speed “B14”.
In the case of the speed change transmission apparatus described above, in the course of a speed change from one speed range to another speed range (this will be referred to as “range skipping speed change” hereinafter), especially in the course of switchover of the speed range between the second speed range and the third speed range, there sometimes occurs a momentary power transmission interruption or any other unstable speed change.
More particularly, the range skipping speed change where the speed range is switched over between the second speed range and the third speed range, is effected by switching over all clutches from one of the engaged and disengaged conditions to the other. Further, when there occurs a change in the drive load applied to the speed change transmission apparatus, thus leading to a change in the load applied to the clutch, even if a designed switchover operational force is applied to the clutch, there tends to occur a situation in which the clutch is switched over at a timing different from the designed timing. Further, the greater the number of clutches to be switched over, the greater the number of clutches which are switched over at different timings from the designed timings and the greater the difference in the switchover timings among the clutches. For this reason, in the course of the range skipping speed change where the speed range is switched over between the second speed range and the third speed range, during the period from the completion of switchover of the respective clutches into predetermined operational conditions and the switchover of the speed change output section to the operational condition for realizing the second speed range or the third speed range, a certain clutch may be kept under the disengaged condition for an extended period of time or kept under a slipping condition. Or, an unstable speed change operation may occur in which even after some clutches are operated into predetermined operational conditions, the other clutches have not yet been switched over to predetermined operational conditions, so that the speed change output section is operated into an operational condition for realizing the first speed range or the fourth speed range before being switched over to the operational condition for realizing the second speed range or the third speed range.